Power control is commonly used in communication systems for minimizing transmission power while maintaining the received signal quality at the desired level. In a code division multiple access (CDMA) spread spectrum communication system, since one user's signal contributes to other users' noise, power control is essential to mitigate the near-far problem and improve the system capacity. Furthermore, in order to minimize power consumption while ensuring a specified minimum quality of service (QoS) under varying channel conditions, the power control target, which is typically a threshold for the received signal to interference ratio (SIR), is updated autonomously to adapt to the change of communication environments. The QoS is typically specified in terms of a block error rate (BLER) or a bit error rate (BER). Examples of such communication systems include those operating under the IS-95, IS-2000, UMTS/WCDMA and TD-SCDMA standards.
For example, in a UMTS/WCDMA system (the UMTS/WCDMA standard can be found at http://www.3gpp.org), an open loop power control scheme is used for determining an initial transmission power at the start of a transmission. A closed loop power control scheme is used to adjust the ongoing transmission power to warrant the specified minimum QoS. The closed loop power control scheme includes both an inner loop power control system and an outer loop power control system. The inner loop power control system in a receiver estimates the received SIR and compares it to the power control target SIRtarget. If the estimated SIR is greater than the target SIRtarget, the receiver generates a power down command that is sent to the transmitter. Conversely, if the estimated SIR is lower than SIRtarget, the receiver generates a power up command that is sent to the transmitter. The transmitter then adjusts the transmission power based on the decoded received power control commands. This inner loop power control system operates at a 1,500 Hz update rate. The outer loop power control system uses an algorithm to control SIRtarget by adjusting it such that the specified minimum QoS is achieved at minimum power all the time.
A significant concern in the SIRtarget update algorithm is the resulting power-rise. Power rise is defined as the difference between the actual average transmitted power and the minimum transmitted power required to meet the specified minimum QoS. The smaller (and non-negative) the power-rise, the better the SIRtarget update algorithm for several reasons. A larger power-rise results in reduced system capacity due to the nature of a spread spectrum communication system. This excess transmitted power reduces the battery life for a mobile terminal such as a cellular telephone. The excess transmitted power also produces un-necessary interference to other mobile receivers.
If the transmitted power is lower than that required to warrant the specified minimum QoS, communication will suffer high error rate or even dropouts may occur.
A prior art SIRtarget update algorithm 100 is illustrated in FIG. 1a. In this prior art, a receiver receives a series of data blocks, one block at each time. Each block can be determined as a good block or a bad block based on, for example, the result of a CRC check. Upon decoding the current data block, the block is checked for errors 102. If an error occurs, the SIRtarget update algorithm steps up SIRtarget by an integer multiple K of a fixed increment A as shown by 104. If no error occurs, the SIRtarget update algorithm would step down SIRtarget by the fixed increment A as shown by 106. By using fixed increments, significant overshoot and undershoot occurred. It should also be noted that this prior art SIRtarget update algorithm bases its SIRtarget update on just the current data block. This memory-less operation will produce large power-rise under steady channel conditions when the SIRtarget is expected to be as constant as possible.
An alternative SIRtarget update algorithm is based upon the proportional-integral-derivative (PID) controller as shown in FIG. 1b. This approach filters the difference between the specified minimum QoS (labeled as “Desired QoS”) and the actual QoS and then updates SIRtarget based upon this difference. It should be noted that in this prior art the actual QoS is computed from all the previously received data blocks. Under varying channel conditions, the SIRtarget is expected to track and compensate the change of channel as quickly as possible. This full-memory operation, however, responded slowly to the change of channel, and results in significant overshoot and undershoot, and therefore high power-rise.
Thus there exists a strong need to reduce the power-rise in a power-controlled communication system by using variable step-size based on proper length of history in the SIRtarget update algorithm.